Inorg. Chem. 1992, 31, 3173-3175
Infrared Spectra of P&o, P& Argon
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and P4S7 in Solid
Lester Andrews,*l+Craig "bompsolqt and Michel C. D6marq***
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Chemistry Department, University of Virginia, Charlottesville, Virginia 22901, and Laboratoire de Chimie Organique, Institut National des Sciences Appliqubs, 20 Avenue Albert Einstein, F6962 1 Villeurbanne, France
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Received May I O , 1992
Introduction
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The recent observation by Andrews and co-workers of P4S9 absorptions in the infrared spectra of P ~ S Isublimates O trapped in a matrix of solid argon led these authors to conclude that some decompositionmust accompany evaporation of solid P4s10 at temperatures as low as 175 OC.I This statement is in conflict with the observation of D6marcq and Lamalle that pure solid P ~ S I O sublimes undissociated under reduced pressure below about 260 OC.2 In order to resolve this disagreement, new matrix experiments were done with freshly purified samples of P4S10, P4S9, and P4S7 and IR spectra were recorded.
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Experimental Section The matrix isolation technique was the same as reported previous1y.l The original Fluka sample of P&IOwas subjected to the purification procedure described by D6marq.3 Another sample of P&o, purified in the same way, was supplied by D t m a r q , as well as specimens of pure P&g3and P&7. The samples were loaded into a stainless steel Knudsen cell, which was evacuated and heated to 70-80 OC under vacuum overnight before use.
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Figure 1. Infrared spectra in the 780480-cm-I region for phosphorus sulfides in solid argon at 12 f 1 K (a) P & Jvapor a t 170 3 OC d e p o s i t e d with argon for 3 h; (b) P4S9 vapor at 178 3 OC d e p o s i t e d with argon for 5 h; (c) P4S7 vapor a t 205 3 OC d e p o s i t e d with argon for 4 h.
Figures la and 2a illustrate the matrix infrared spectra of the vapor effusing from solid P ~ S Iat O 170 OC and trapped in solid argon. The spectra were virtually identical for the purified Fluka sample, the sample provided by Dhmarcq, the sample collected in a second subsequent matrix experiment, and a sample evaporated from a quartz Knudsen cell. Spectra similarly obtained with P4S9 and P4S7 are shown in Figures lb/2b and lc/2c, respectively. The above spectra are dominated by the characteristic absorption peaks of P ~ S Iat O 707.8 and 540.8 cm-I, of P4S9 at 71 1.3,553.0, and 499.0 cm-I, or of P4S7at 725.7 and 717.2 cm-I. Assignment proposals are listed in Table I and correlated with literature data in terms of frequency shifts A(matrix4rystal) and A(matrix-solution). In line with a recent low-temperature Raman study on P4Slo,Io all these shifts, but one, are positive. They are largest for P=S stretching vibrations, especially as regards the shifts A(matrix-crysta1) for P4S7, while the shift A(matrix-solution) of this sulfideappears normal. This anomaly t University of Virginia.
t Institut National des Sciences Appliquh. (1) . . Andrews. L.; Reynolds, G. G.; Mielke. Z.; McCluskey, M.Inorg. Chem. 1990. 29, 5222. (2) Dtmarq, M. C.; Lamalle, M. Rev. Chim. Miner. 1983, 20, 396. (3) Dtmarw, M. C. J . Chem. Soc., Dalton Trans. 1990. 35. (4) Gardner; M. J. Chem. Soc., Dalton Trans. 1973,691. ( 5 ) Wang Zongming; Wang Xiqing; Lu Wanzhen. Proceedings of the ~
(6) (7) (8) (9)
(10)
VIIth International Conference on Raman Spectroscopy; Murphy, W. F., Ed.; North-Holland Publishing Co.: Amsterdam, 1980; p 132. Somer, M.; Bues, W.; Brockner, W. Z . Naturforsch. 1983, 384 163. Dtmarq, M. C. Phosphorus Sulfur 1981, I I , 6 5 . Dtmarq, M. C. Unpublished results. Mielke, Z.; Brabson, G. D.; Andrews, L. J. Phys. Chem. 1991,95, 7 5 . Rollo, J. R.; Burns, G. R.J . Phys. Chem. Solids 1989, 50, 399.
0020-166919211331-3173$03.00/0
might originate in the existencein solid P& of specificP=S* *PI11 intermolecular association.llJ2 The assignment of the 750.5-cm-I band to P~SS, assumed to have a S2PSPS2 structure,' finds support in the infrared spectrum of the dithioxophosphorane2,4,6-But$sH2PS2,13 which displays a strong bandat 788 cm-I, assignabletoantisymmetric v[P(=!3)2]. Other high-frequency absorptions at 765.8 and 748.3 cm-l are similarly believed to belong to P(==S)2 molecular species.'
Discussion
P&. The appearance in Figures la and 2a of P4S9 bands at 711.3, 553.0, and 499.0 cm-I confirms that P ~ S Itends O to desulfurize on sublimation under vacuum, at temperatures as low as 170 OC; the yield of P&, lower than in previous work,1J4 was estimated at about 10% under the assumption of comparable extinction coefficients for the 707.8- and 71 1.3-cm-I bands. In order to explain the initial first-order kinetics observed for the redox dissociationof P ~ S IinOsolution, D6marcq has suggested a mechanism involving a rate-controlling fragmentation to PZSS (eq 1) followed by a fast sulfur abstraction from P ~ S Iby O P2sS (eq 2).3 In the vapor phase, step la is expected to take place as well, but due to the low vapor p&sure of solid P4S10 (1.1 X (11) Brevard, C.; Dtmarq, M.C. Chem. Phys. Lctt. 1981, 82, 167. (12) DCmarq, M. C. J . Phys. Chcm. 1990, 94,6514. (1 3) Personal communication from Dr. J. Navcch to M.C.D., who expresses his thanks. (14) The larger yields of P& found previously1 are explained by impurity of the commercial sample of P4&0.~-*5 (15) Dtmarq, M.C. Ind. Eng. Chem. Res. 1991,30, 1906.
0 1992 American Chemical Society
Notes
3174 Inorganic Chemistry, Vol. 31, No. 14, 1992
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Figure 2. Infrared spectra in the 580480-cm-' region for phosphorus sulfide samples described in Figure 1. Table I. Infrared Absorptions of Matrices Prepared by Condensation of P4S10, P s g , or P4S7 Vapors with Argon at 12 f 1 K freq, re1 intensity cm-l P ~ S I OP4S9 P4S7 765.8 750.5 748.3 733.1 728.0 725.7 717.2 711.3 708 707.8 704.3
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P4S9-t P2Sn+l( n = 5,6, ...I (2) abstraction step (eq 2) should be much slower than in solution, P4S10
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shift (A)/ cm-' matrix- matrixcrystal solution
so that the (P&)/(P4Sl0) concentration ratio should rise to relatively high levels (as denoted by the absorption at 750.5 cm-I in Figure la), while the time required for this ratio to reach its maximum (steady state) should be increased. Also, the overall reaction rate in the vapor is not necessarily controlled solely by the step la, as in solution.' The discrepancy between the present experimental results and those of Demarq and Lamalle2may be rationalized in terms of effective heating time of the vapor. In the first case, P ~ S I O molecules are allowed to react during their stay inside the Knudsen cell (kept at a constant temperature), before effusing through the orifice (1 mm); in the latter case, they travel directly to the cold finger along a temperature gradient17 and react only negligibly. The yield Of P4s9 is expected to be particularly sensitive to the heating time (t) at the very onset of the reaction (before the steady state is reached), since then this yield is approximately proportional to t2. Consistent with eqs 1 and 2, it is noteworthy that none of the S, species (x I 8) recently identified by their infrared spectra in argon matrices'* are detectable in Figures la and 2a, as well as in previous matrix spectra.' Since we lack information on the vibrationalbehavior of polysulfides PIS,( n > 5), their localization in Figures la and 2a can only be speculative; small peaks at 700.2 and 706.1 cm-I, absent in Figure lb, are possible candidates. PA. The appearance in Figures l b and 2b of bands at 707.8, 540.8, and 717.2 cm-' is a sign that solid P4S9 disproportionates to P4Slo + P a 7 on evaporation at 178 "C,as also previously observed for the molten sulfide (about 20% decomposition in 5 s at 260-270 "C).I9 The height of the P2s5 peak at 750.5 cm-' in Figure l b proves that this species does not solely arise from the dissociation of the byproduct P4S10 but also arises from that of P4S9 itself (eq 3). This fragmentation, followed by steps 4 and 1b, could be the way
P4S9 * P,Ss
+ P2S4
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0 Data from refs 4-8. Appears in PHs/Ss discharges, probably due to the PzS5 hydrolysis product, HSPS2.I cThis shoulder may be the 748.9-cm-' band previously assigned to P2S4;'.9however, the absorbance ratio of 748.3 to 750.5 cm-l is about the same in Figure la,b and is seemingly insensitive to superheatipg,' which suggests that the former is a matrix site splitting of the latter. The absence of absorption at 540.8 cm-' in Figure 2c demonstrates that the 708-cm-I band in Figure IC is not due to P4Slo impurity and is distinct from the 707.8-cm-l band of the latter.
Torr at 170 0C7,16), the recombination reaction (eq lb) and the
the disproportionation to P4S10 + P4S7 takes place. A driving force for the above mechanism would certainly be the irreversibility of the second part of step 4, a consequence of the instability of P4Ss;I9 this would explain the smallness (or absence; see Table I) of the P2s4 band at or near 748.3 cm-I in Figure lb. PA. It is clear from Figures ICand 2c that solid P4S7 sublimes undissociated at 205 "C. This observation is in agreement with known behavior of this compound, which was found by 31P NMR spectroscopy to withstand melting (5 min at 330 "C) without decomposition.8 P&. One argument in favor of the identificationof the 750.5and 508.4-cm-1 absorber as P2s5 is the abundance of the P2S5+ ion in the mass spectra of P4S10.' This is contradicted by the work of Muenow and Margrave, who inferred from the high appearance potential of this ion (14.2 eV, compared with 9.6 eV for P4Sl0+)that the P2S5+ signal is not a molecular peak.20 The disagreement may be only apparent however, as the data of Muenow et al. may simply mean that the ionization of molecular P2s5 requires a high potential. Indeed, while in monothioxophospho(17) In one sublimation experiment performed at about 260
O C under 0.8 Torr,2 the effective residence time of the vapor in the hot zone was estimated at less than 0.1 s. (18) Brabson, G. D.; Mielke, 2.;Andrews, L. J. Phys. Chem. 1991, 95, 79. (19) Barieux, J.-J.; DCmarq, M. C. J . Chem. Soc., Chem. Commun. 1982,
176. (16) DCmarq, M. C. J . Muter. Sci. Let?.,in press.
(20) Muenow, D. W.; Margrave, J. L. J. Inorg. Nucl. Chem. 1972,34,89.
Notes ranes Z#=S, lone pelectron pairs on S back-coordinate chiefly intovacant d orbitais on sp3P, in dithioxophosphoranes Z P ( S j 2 they must also overlap with Pr orbitals On sP2 Since Pr-Pr bonds are stronger than Prd* bonds, the energy Of P lone pairs On in (~)2PSP(ES)2shou1dbe10wered and--bearing in mind that lone pairs on S are major componentsof the HOMO in P=S compounds (see for instance ref 23)-the first ionization potential should be increased. (21) The dithioxophosphorane 2,4,6-But,CsHzPSz,regarded as a model for P2Ss, involves a planar CPSZmoiety (S-M= 126°).22 A planar and intramolecular F=S. * .S=P structure, involving bent P-S-P interactions, was similarly suggested for P2Ss.S (22) Appel, R.;Knoch, F.; Kunze, H. Angew. Chem. 1983,95, 1008. (23) Cowley, A. H.; Lattman, H.; Montag, R. A.; Verkade, J. G. Inorg. Chem. 1984, 23, 3318.
Inorganic Chemistry, Vol. 31. No. 14, 1992 3175
Conclusions The present infrared matrix experiments confirm that P4SIo undergoes redox dissociation in the vapor phase, with OfP4S9,at temperatures as low as 170 OC, Previouscontradictory results are ascribed to shorter effective heating time of the vapor. The vapor of P4S9 was similarly shown to disproportionate to p4s, + p4sl0at 178 OC, and p4s7was shown sublime undissociated at 205 "C. Evidence was obtained for the fragmentations of P4S10 to P2Ss and Of P4s9 to P2ss + P2s4 in the gas phase. The latter species could act as intermediates in the redox dissociation process of P4SIO and P4S9.
Acknowledgment. L.A. and C.T. gratefully acknowledge financial support from NSF Grant CHE 88-20764.
